(1) Field of the Invention
This invention relates to photopolymerizable compositions, and more particularly to photopolymerizable compositions containing bis(substituted amino)sulfides as inhibitors of thermal polymerization.
(2) Description of the Prior Art
Photopolymerizable ethylenically unsaturated compositions, particularly monomer/binder systems, are useful in the preparation of image-bearing objects, such as photoresist coatings, lithographic films, etc. Compositions containing a polymerizable monomeric compound and a radiation sensitive, free-radical generating system have variable shelf life or stability, especially at elevated temperatures. Inhibitors have been added to these compositions to improve stability and provide photopolymer images of superior properties as shown in Belgian Pat. Nos. 809,915 and 818,371 wherein certain nitroso dimers are incorporated with the polymerizable monomer and the free-radical generating system. These nitroso dimers are not free-radical inhibitors, but dissociate to inhibiting mononitroso species, particularly at higher temperatures where polymerization normally occurs.
The term "bis(substituted amino)sulfide", as used throughout the specification and claims, is intended to cover both monosulfides and polysulfides. These sulfides have been used heretofore in photochemical compositions. For example, in German Pat. No. 884,602 bis(substituted amino)sulfides are used to stabilize photographic silver halide/gelatine emulsions in combination with a sulfinic or selenic acid or salt. These sulfides have also been used with polymers as curing or vulcanizing agents and as stabilizers of polyurethane elastomers. However, it has not heretofore been suggested that they be used as thermal polymerization inhibitors for photopolymerizable compositions.
Disulfides in general are known to react with radicals to produce thioethers and thiyl radicals [Danen & Newkirk, J. Am. Chem. Soc. 98, 516 (1976); and Pryor & Platt, J. Am. Chem. Soc. 85, 1496 (1963)]. If this type of reaction were to occur with bis(substituted amino)sulfides, an aminothiyl or thionitroxide radical would be formed. These radicals are known to prevent free-radical induced chain propagation [see Maillard & Ingold, J. Am. Chem. Soc. 98, 520 (1976)].